The present invention relates in general to a field-emission display and a method of fabricating the same, and more particular, to a method and a structure that introduce a fourth electrode (converging electrode) to a conventional triode field-emission display.
Flat panel displays such as field-emission display (FED), liquid crystal display (LCD), plasma display panel (PDP) and organic light emitting diode display (OLED) have become more and more popular in the market. Lighter and thin are the common features of flat panel displays. According to specific characteristics such as dimension and brightness, some of the above are suitable for small dimension display panel such as cellular phone and personal data assistant (PDA), some are suitable for medium or large size display such as the computer and television screens, or some are even suitable for ultra-large size display such as the outdoor display panel. The development trend of various displays is to obtain high image quality, large display area, low cost and long life time.
The field-emission display is a very newly developed technology. Being self-illuminant, such type of display does not require a back light source like the liquid crystal display. In addition to the better brightness, the viewing angle is broader, power consumption is lower, response speed is faster (no residual image), and the operation temperature range is larger. The image quality of the field-emission display is similar to that of the conventional cathode ray tube (CRT) display, while the dimension of the field-emission display is much thinner and lighter compared to the cathode ray tube display. Therefore, it is foreseeable that the field-emission display may replace the liquid crystal display in the market. Further, the fast growing nanotechnology enables nano-material to be applied in the field-emission display, such that the technology of field-emission display will be commercially available.
FIG. 1 shows a conventional triode field-emission display, which includes an anode plate 10 and a cathode plate 20. A spacer 14 is placed in the vacuum region between the anode plate 10 and the cathode plate 20 to provide isolation and support thereof. The anode plate 10 includes an anode substrate 11, an anode conductive layer 12 and a phosphor layer 13. The cathode plate 20 includes a cathode substrate 21, a cathode conductive layer 22, an electron emission layer 23, a dielectric layer 24 and a gate layer 25. A potential difference is provided to the gate layer 25 to induce electron beam emission from the electron emission layer 23. The high voltage provided by the anode conductive layer 12 accelerates the electron beam with sufficient momentum to impinge the phosphors layer 13 of the anode plate 10, which is then excited to emit a light. To allow electron moving in the field-emission display, the vacuum is maintained at least under 10−5 torr, such that a proper mean free path of the electron is obtained. In addition, contamination and poison of the electron emission source and the phosphors layer have to be avoided. Further, the electron emission layer 23 and the phosphors layer 13 have to be spaced from each other by a predetermined distance for accelerating the electron with the energy required to generate light from the phosphors layer 13.
The conventional electron emission layer is typically in the form of a spike structure (as shown in FIG. 1) or a Spindt type structure. The latter structure includes a spike structure formed by thin-film process or photolithography process. By further development of thin-film process, various Spindt type field-emission display has been proposed and improved. The electron beam induced by electric field at the spike normally propagates in a curve with a small radius. Control electrodes in various configurations are introduced in the conventional field-emission display to correct the cross section of the electron beam or to guide the electron beam along the correct path to impinge the phosphors at the correct position. Therefore, the conventional field-emission display requires the spike structure of the electron emission source, the electron configurations, and the process of thin-film, photolithography or micro-electro-machining. These requirements hinder the development of field-emission display since sixties.
Recently, a carbon nanotube has been proposed by Iijima. Having high aspect ratio, high machine strength, high chemical resistance, abrasion resistance, low threshold electric field, the carbon nanotube has been popularly studied and applied as an electron emission source. As known in the art, the field electron emission is facilitated by applying a high electric field to a surface of a material to reduce the thickness of energy barrier of the material, such that electron can be ejected from the surface of the material to become a free electron according to quantum-mechanical tunneling effect. The current of the field electron emission can be increased by reducing the work function of the material surface. As the free electron is generated by the electric field, a heat source is not required, and the field electron emission apparatus is sometimes referred as a cold cathode.
The carbon nanotube has been continuously improved and applied to continuously enhance electron emission of a field-emission display. Currently, the carbon nanotube can be fabricated by a thick-film process (such as screen printing or spray printing). Referring to Chinese (Taiwanese) Patent Publication No. 502495, the carbon nanotube can be directly patterned on the cathode conductive layer 22 to form the electron emission layer 23 thereon. Thereby, the conventional triode field-emission display is not limited to the high-cost thin-film process. The carbon nanotube electron emission source provides a high electron emission efficiency (with a current density of 10 μA/cm2 and a threshold voltage of 1.5 V/μm, and a current density of 10 mA/cm2 under an electric field of 2.5 V/μm) which achieves perfect dynamic display effect with a lost cost driving circuit. Even so, each electron emission source unit is constructed of a plurality of carbon nanotubes, such that the electron beam generated thereby within the distance between the anode and the cathode is similar to that generated by the spike field-emission source. Therefore, the cross section of the collected electron beam 26 diverges while approaching the anode as shown in FIG. 2. The longer the distance is, the larger the cross section of the electron beam 26 is. It is possible that the cross section is larger than the luminescent area of the phosphors layer 13, or the diffused electron beam 26 might impinge the neighboring phosphors layer 13 to affect the color purity or image resolution.
To resolve the color purity or image resolution issue, the area of the electron emission source is reduced or partitioned into a plurality of smaller units, such that the electron beam 26 generated thereby is similar to the area of the corresponding phosphors layer 13 excited thereby. However, the reduction in cross section results in a lower efficiency of electron emission or reduced unit area of the corresponding phosphors layer 13, such that the space between neighboring phosphors layer 13 is increased, and the image resolution is degraded.
Another method to resolve the issue is to provide an adjustable voltage between the gate electrode 25 and the cathode conductive layer 22. In addition to electron drainage, the gate layer 25 can also control the cross section of the electron beam by adjusting the voltage. This type of design results in a lower efficiency of electron generation and a more complex circuit design. Further, the response time of the picture is increased, and the image quality is lowered.
The third method to resolve the above issue includes forming one or more than one set of control electrode between the cathode and the anode. The control gate provides a converging voltage or bias voltage to confine the cross section of the electron beam or deflect the electron beam, such that the electron beam can impinge the phosphors layer 13 at the predetermined position. However, such type of design requires complex fabrication process such as thin-film and lithography process and cannot meet with the requirement of large area display and mass production.